This invention relates to imaging systems, and more particularly, to improved electrostatographic developing materials, their manufacture and use.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layers exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may substantially be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if the powder image transfer step is not desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Several methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely divided toner particles electrostatically coated thereon is conveyed to and rolled or cascaded across the electrostatic image bearing surface. The composition of the carrier particles is so selected as to triboelectrically charge the toner particles to their desired polarity. As the mixture cascades or rolls across the latent image bearing surface, the toner particles are electrostatically deposited and secured in positive development processes to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background areas are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier and excess toner are then recycled. This technique is extremely good for development of line copy images.
Another method for developing electrostatic images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner particles and magnetically attractable carrier particles are carried by a magnet. The magnetic field of the magnet causes alignment of the magnetically attractable carrier particles into a brushlike configuration. This magnetic brush is engaged with the electrostatic image bearing surface and the toner particles are drawn from the brush to the latent image by electrostatic attraction.
Still another technique for developing electrostatic latent images is the "powder cloud" process as disclosed, for example, by C. F. Carlson in U.S. Pat. No. 2,221,776. In this method, a developer material comprising electrically charged toner particles in a gaseous fluid is passed adjacent the surface bearing the electrostatic latent image. The toner particles are drawn by electrostatic attraction from the gas to the latent image. This process is particularly useful in continuous tone development.
Other development methods such as "touchdown" development as disclosed by R. W. Gundlach in U.S. Pat. No. 3,166,432 may be used where suitable.
Generally, commercial electrostatographic development systems utilize automatic machines. Since automatic electrostatographic imaging machines should operate with a minimum of maintenance, the developer employed in the machines should be capable of being recycled through many thousands of cycles. In automatic xerographic equipment, it is conventional to employ an electrophotographic plate which is charged, exposed and then developed by contact with a developer mixture. In some automatic machines, the toner image formed on the electrophotographic plate is transferred to a receiving surface and the electrophotographic plate is then cleaned for reuse. Transfer is effected by a corona generating device which imparts an electrostatic charge to attract the powder from the electrophotographic plate to the recording surface. The polarity of charge required to effect image transfer is dependent upon the visual form of the original copy relative to the reproduction and to the electroscopic characteristics of the developing material employed to effect development. For example, where a positive reproduction is to be made of the positive original, it is conventional to employ a positive corona to effect transfer of a negatively charged toner image to the recording surface. When a positive reproduction from a negative original is desired, it is conventional to employ positively charged toner which is repelled by the charged areas on the plate to the discharged areas thereon to form a positive image which may be transferred by negative polarity corona. In either case, a residual powder image usually remains on the image after transfer. Because the plate may be reused for a subsequent cycle, it is necessary that the residual image be removed to prevent "ghost images" from forming on subsequent copies and toner film from forming on the photoreceptor surface. In a positive to positive reproduction process described above, the residual powder is tightly retained on the plate surface by a phenomenon not fully understood which prevents complete transfer of the powder to the support surface, particularly in the image area. Incomplete transfer of toner particles is undesirable because image density of the ultimate copy is reduced and highly abrasive photoreceptor cleaning techniques are required to remove the residual toner from the photoreceptor surface. This imaging process is ordinarily repeated for each copy reproduced by the machine any time during the reusable life of the developer and the electrophotographic plate surface.
Various electrostatographic plate cleaning devices such as the "brush" and the "web" cleaning apparatus are known in the prior art. A typical brush cleaning apparatus is disclosed by L. E. Walkup et al, in U.S. Pat. No. 2,832,977. The brush type cleaning means usually comprises one or more rotating brushes, which remove residual powder from the plate into a stream of air which is exhausted through a filtering system. A typical web cleaning device is disclosed by W. E. Graff, Jr. et al in U.S. Pat. No. 3,186,838. As disclosed by Graff, Jr. et al., removal of the residual powder on the plate is effected by passing a web of fibrous materials over the plate surface. Another system for removing residual toner particles from the surface of a photoreceptor comprises a flexible cleaning blade which wipes or scrapes the residual toner from the photoreceptor surface as the surface moves past the blade.
Unfortunately, the foregoing cleaning systems do not effectively remove all types of toner particles from all types of reusable photoreceptors. This is not a shortcoming of the cleaning system, but a shortcoming of particular toners used in conjunction with particular photoreceptors. If a particular toner would not tend to form an adherent residual film on a particular photoreceptor, the cleaning systems described would effectively remove all residual toner. However, many commercial toners of their very nature do tend to form a residual film on reusable photoreceptors. The formation of such films is undesirable because it adversely affects the quality of undeveloped and developed images. The toner film problem of these particular toners is acute in high speed copying and duplicating machines where contact between the developer and the imaging surface occurs a great many more times and at a higher velocity than in conventional electrostatographic systems. Ultimately, the toner buildup becomes so great that effective copying or duplicating is impaired. As a result, more stringent means, e.g. solvent removal, are necessary to remove this type of film. Frequent shutdown of the apparatus, in order to clean the surface of the photoreceptor is obviously undesirable since the machine is taken out of commission and repeated techniques of this type wear down the photoreceptor surface.
Thus, there is a continuing need for a technique for eliminating the buildup of toner film on the surface of a photoreceptor. Electrostatographic systems and, in particular, the imaging, developing and cleaning aspects of such systems would be significantly advanced if the foregoing problems were effectively overcome.